Electro-optical lenses that utilize birefringent liquid crystal to alter their optical power are known. They have the inherent advantage over conventional glass or plastic optical lenses of being able to alter their optical power by the judicious application of an electric field. One drawback of existing liquid crystal electro-optic lenses is that the number of optical powers a single lens can generate is presently limited.
One basic structure of electro-optic liquid crystal lenses is that of a thin layer of liquid crystal sandwiched between two transparent substrates. Onto the inner surfaces of each substrate, a transparent metallic electrode structure is formed. An alignment layer is formed on top of the electrode layers to establish a specific orientation of the liquid crystal molecules when there is no electric field present. An electric field is established across the liquid crystal layer when voltage is applied to one electrode layer and an electric potential is created between the electrodes. If the electrode structure is patterned, a gradient in the field is created that gives rise to a gradient in the index of refraction of the liquid crystal layer. With proper design of the electrode structure and the applied voltages, an electro-optic lens can be fabricated.
Electro-optic liquid crystal lenses have been designed and fabricated that utilize electrode structures to generate several optical powers with a single lens.
The basic structure of a spherical electro-optic liquid crystal lens is that of a circular ring electrode design, where the transparent electrodes on one or both substrates consist of toric rings, electrically insulated from adjacent neighboring rings. Previous designs of these lenses are restrictive in the sense that the ring electrode widths and spacing often determine the optical power of the lens. However, if a very large number of very narrow electrodes could be fabricated and addressed individually, theoretically, a very large number of optical powers could be generated by such a lens.
Considering that the optical phase change between each adjacent electrode should be less than about ⅛ of a wave and that the total phase change across a lens might be as high as 100 waves, it first appears that an electrode structure consisting of hundreds of rings addressed by hundreds of input connections to the device might be required for continuous tuning. This is not an acceptable solution, however, since the photolithography needed to create such an electrode structure would be daunting. Moreover, fabricating the buss structure to connect and electrically address each electrode would be an overwhelming task and make the resulting device extremely complex and unwieldy.
The use of phase-wrapping can help mitigate the problem of fabricating hundreds of input connections to the lens. It has been previously shown in “Liquid Crystal Based Electro-Optic Diffractive Spectacle Lenses and Low Operating Voltage Nematic Liquid Crystals” by Joshua Naaman Haddock, a Dissertation submitted to the Faculty of the College of Optical Sciences in partial fulfillment of the Requirements for a Degree of Doctor of Philosophy in the Graduate College of the University of Arizona in 2005, that electrodes can be grouped in such a way that the phase change over one group is approximately one wave. Thus, the number of input connections is limited to the number of rings in each group. However, this scheme only provides high efficiency if the phase change across each group of electrodes is very close to a multiple of one wave. Thus, the phase change across each electrode group cannot be changed in a continuous manner, and as a result, the lens cannot be continuously tuned to multiple powers.
U.S. Publication No 2008/0212007 relates to an electro-optic device comprising a liquid crystal layer between a pair of opposing transparent substrates; a resistive patterned electrode set positioned between the liquid crystal layer and the inward-facing surface of the first transparent substrate; and a conductive layer between the liquid crystal layer and the inward-facing surface of the second transparent substrate, wherein the conductive layer and resistive patterned electrode set are electrically connected, and wherein said resistive patterned electrode set comprises one or more electrically-separated electrodes, wherein the desired voltage drop is applied across each electrode to provide the desired phase retardation profile.